Field Effect Transistors (FET) can be used as high frequency electromagnetic waves detectors. At the interface between the layers of a FET, notably between the metal and semiconductor layers, the crystal structure of the semiconductor material shows defects and irregularities in which results mobility of charge carriers.
At the interface, the tightness of the bounds between the atoms and the electrons is weaker. Thus, the mobility of the charge carriers is greater.
The resulting mobility of the charge carriers is referred to as “gas of carriers” or “plasma”. The oscillations that can appear within the plasma are referred to as “plasmons”.
When a high frequency electromagnetic wave reaches a FET, a voltage may appear between the source and the gate (or the drain and the gate) of the transistor. The voltage appears when the gate shows asymmetry as compared to the source (or the drain), i.e. when the gate is not at the same electrical potential as the source (or drain). This results in a voltage appearing between the two nodes which is proportional to the power of the incoming electromagnetic wave, as the result of the charge carriers over the channel resistance.
Detection of electromagnetic terahertz waves by FETs has been used in the field of spectroscopy for the reason that terahertz waves well penetrate some materials. Document Knap et al. “Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications” (Journal of Infrared, Millimeter, and Terahertz Waves, December 2009, Volume 30, Issue 12, pp 1319-1337) provides a review of the theory and experimentations that have been conducted for spectroscopy so far.
However such applications only aim at obtaining a DC voltage between drain and source when the FET is illuminated by electromagnetic waves. The techniques used are not convenient for transmitting data.
High frequencies are generally underused whereas they provide a large bandwidth modulation potential. Indeed, the ratio between the carrier frequency and the modulation bandwidth is easier to optimize when comprised between 10 and 100. For example, a 300 GHz carrier should provide a 30 GHz modulation bandwidth. The prior art techniques do not provide such modulation bandwidth. Document Blin et al. “Plasma Waves Detectors for Terahertz Wireless Communications” (IEEE electron device letters, vol. 33, N10, October 2012) discloses the first experiment that has been conducted for determining the modulation bandwidth achievable at room temperature with a FET. A modulation bandwidth of 8 GHz has been attained.
Document Ojefors et al. “A 0.65 thz FOCAL PLANE Array In a Quarter Micron CMOS Process technology” (IEEE Journal of Solid State Circuits, Vol. 44, No 7, July 2009) discloses the use of a FET operated at zero DC drain voltage. The FET behaves as a resistive mixer with a high linear resistance. Self-mixing is achieved by inserting a capacitor between the gate and the drain of the FET. The gate is biased through the use of the virtual ground of the receiving antenna. It is to be reminded that a virtual ground is a node of a circuit where the alternative currents cancel each another.
However, while the arrangement disclosed is convenient for spectroscopy, it cannot be used in communication applications.
Also, the transistor cannot be used with an optimal bias. Therefore the conversion of the terahertz power into voltage may not be optimal.
Another drawback is that the transistor behaves as a passive mixer. There is no amplification performed.
Another drawback is that adding some conducting element to the virtual ground of the antenna to bias the gate modifies the geometry of the antenna, and further modifies its electrical characteristics. For an antenna of small dimensions adapted to terahertz frequencies, the size of the conducting element surface is comparable to the size of the antenna surface. This renders the design of the antenna impractical.
The capacitor inserted alleviates the modulation bandwidth of the terahertz carrier.
Thus, there is a need for enhanced FET-based detectors that can be used in the wireless communication field.
The present inventors have already disclosed in patent application GB 1313912.6, a detector comprising an antenna and a FET transistor. The gate of the transistor is connected to a virtual mass in order to obtain a desired polarization of the transistor gate.
The inventors have now provided enhancements to the detector by coming up with a particular design of the transistor that makes it possible to obtain optimal adaptation between the antenna and the transistor.